The filtration operations such as removal of bacteria and turbidity particles using porous membranes such as microfiltration membranes and ultrafiltration membranes are put to practical uses in a wide variety of fields such as the automotive industry (systems for recovery and reuse of electrodeposited paints), semiconductor industry (production of ultra-pure water), and medicine and food industries (removal of bacteria, enzyme purification). Particularly, application to the field of water supply in which potable water and industrial water are prepared by removing the turbidity of river water, etc. and application to the field of sewage in which sewage purification is carried out by removing the turbidity of sewage (secondarily treated sewage) have recently been intensively attempted. In order that the membranes are widely employed in these fields, they are required to be inhibited from staining (clogging) with organic substances, etc.
As materials of the membranes, there are used various materials such as cellulosic materials, polyacrylonitrile-based materials, and polyolefinic materials. Among them, polyvinylidene fluoride is high in strength and heat resistance, and, furthermore, is high in water resistance because its skeleton is hydrophobic and, thus, is suitable as materials of water filtration membranes and is therefore expected to be useful.
As a method for producing polyvinylidene fluoride membranes, U.S. Pat. No. 5,022,990 proposes a method for producing hollow fiber membranes which comprises melt kneading polyvinylidene fluoride, an organic liquid and an inorganic powder, then causing micro-phase separation by cooling and extracting the organic liquid and the inorganic powder. Moreover, WO91/172204 discloses a method for producing hollow fiber membranes comprising polyvinylidene fluoride and a solvent system.
In general, it is known that when filtration of raw water containing turbidity components in large amounts is continued, deposits remaining on the surface of the membranes or in the inside of the membranes which have not been filtered off result in a new filtration resistance to cause deterioration of filtration performance. Therefore, there are employed such means as flushing method according to which the filtration is suspended and the deposits are stripped off by a high speed water stream during the filtration operation, air scrubbing method according to which the deposits are stripped off by applying bubbles to the membrane, back washing method according to which the membranes are washed by reversing the direction of filtration, and the like. Another method is to carry out a periodical cleaning with chemicals to maintain the filtration performance at a high level. The flushing method and air scrubbing method are high in cleaning effect of membranes, but they apply a great load to membranes, which is apt to cause rupture of the membranes, and, furthermore, in the case of conventional membranes, even if these cleaning method are employed, considerable stains are deposited on the membranes (clogging of the membranes) with lapse of time. Thus, satisfactory water permeation performance has not necessarily been obtained.
The object of the present invention is to provide a method for stably and satisfactorily producing polyvinylidene fluoride hollow fiber membranes which have dense pores and high water permeation performance and are excellent in endurance and stain resistance, and are suitable for filtration uses such as removal of turbidity.